Hydraulic system for displacing a load with automatic hydrostatic balancing

ABSTRACT

A variable-displacement pump is connected to a motor that rotates a winch for lifting a load. The torque that the suspended load exerts on the winch is detected as is the pressure in the high-pressure line between pump and motor. The output of an auxiliary pump is diverted from a reservoir into this highpressure line to the extent necessary to support the load hydrostatically before the brake on the winch may be released. The hydraulic motor is operated above its lower settings by allowing slip in a differential between it and the winch so that uniform and very slow displacement rates for the load may be achieved.

United States Patent [1 1 Me'tailler [451 Feb. 11, 1975 1 1 HYDRAULIC SYSTEM FOR DISPLACING A LOAD WlTH AUTOMATIC HYDROSTATIC BALANCING [75] Inventor: Roger Mtailler, Grenoble. France [73] Assignee: Societe Anonyme Richier, Paris.

France {22] Filed: July 30, 1973 [21] Appl. No.: 383,534

[30] Foreign Application Priority Data July 28, 1972 France 72.27952 [52] US. Cl 60/468, 60/327, 60/905,

7 60/D1G. 2 51 Int. Cl. Fl5b 20/00, Fl6h 39/46 [58] Field of Search 60/325, 390, 391, 428, 60/435, 441, 442, 451. 452, 459, 468, 905,

[56] References Cited UNITED STATES PATENTS Link 60/468 Huf 60/441 X Kruschc 60/905 X Primary E.t'amim'rEdgar W. Geoghegan Attorney. Agent. or Firm--Karl F. Ross; Herbert Dubno [57] ABSTRACT A variable-displacement pump is connected to a motor that rotates a winch for lifting a load. The torque that the suspended load exerts on the winch is detected as is the pressure in the high-pressure line between pump and motor. The output of an auxiliary pump is diverted from a reservoir into this highpressure line to the extent necessary to support the load hydrostatically before the brake on the winch may be released. The hydraulic motor is operated above its lower settings by allowing slip in a differential between it and the winch so that uniform and very slow displacement rates for the load may be achieved.

15 Claims, 8 Drawing Figures PATENTED E H r SHEET 2 OF 2 I IIIII I HYDRAULIC SYSTEM FOR DISPLACING A LOAD WITH AUTOMATIC HYDROSTATIC BALANCING FIELD OF THE INVENTION The present invention relates to a hydraulic system for displacing a load. More specifically, this invention concerns a system for raising and lowering a load.

BACKGROUND OF THE INVENTION A typical hydraulic system as used in a winch for a crane or the like comprises a variable-displacement hydraulic pump connected via a pair of lines to a fixeddisplacement hydraulic motor which is connected to the load. The-rate of displacement and the torque exerted by this motor is controlled by varying the displacement of the pump. A brake is provided on the motor or between the motor and the load to arrest the load, locking it in place without the use of the hydraulic motor.

In such an arrangement, one of the most difficult problems is starting the load from a stop when, for instance, it is hanging and blocked in place by the brake. As a general rule, the operator of the apparatus estimates the approximate weight of the load, then sets the lift lever which operates the pump at what he considers the appropriate level and slowly releases the brake. Further instantaneous compensation is required as the brake releases to make up for any miscalculations. The particular operation requires considerable skill, and even the most skillful operator often misjudges and damages the item being lifted or lowered. The same problem exists in a car having an automatic transmission when it is started on a steep grade and the parking brake is released: the driver must jockey the accelerator as he releases the brake.

Another problem in many fluid-power operated systems is that it is difficult to obtain a very slow displacement of the load. The pump powering the system invariably delivers its power in the form of pulses, so at low speed the load is subjected to a jerking, uneven force. In a crane this makes it veryldifflcult to pick up or set down a load carefully, often resulting in damage to the support for the load, e.g., the truck, or danger for the personnel supervising the lifting.

In one known system a simple hydraulic ram has its working chamber connected to the high-pressure line of the hydraulic circuit. The piston rod of this ram acts on a pivoting assembly which is subjected to the torque generated by the load on the hydraulic motor. This assembly pivots when the torque generated by the motor is sufficient to overcome the weight of the load, and actuates a switch to decouple the brake. Thus the operator merely advances the lift lever slowly until the pressure generated by the pump is sufficient to hold up the load, at which instant the brake is automatically released.

The pump is usually an axial-piston pump whose swash plate is displaceable by means of the operators lift lever, as well as through a second hydraulic cylinder. This latter cylinder is cut off at the same instant that the brake is released, so that the swash plate is inclined to an extent necessary to hold up the load, unless the operator expressly overrides it to lower the load. Thus this cylinder remembers the weight of the load.

This system is extremely advantageous in that it allows a load to be arrested in the middle of a lift and locked by means of the brake. The lift stick is then moved along to read into the cylinder operating the swash plate the weight of the load, and the rest of the lift is carried out. Should the brake be applied again, the pump will automatically beset by this cylinder at the proper level for the hydrostatic balancing of the load. Such an arrangement is very useful for loads which have static weight. When, however, a bucket is being dumped or two cranes are being used on the same load, it proves extremely inconvenient as it cannot compensate for a changing load. In this case release of the brake often leads to a sudden jumping or dropping of the load.

OBJECTS OF THE INVENTION It is, therefore, an object of the present invention to provide an improved system for displacing a load.

Another object is the provision of a hydraulic system for displacing a load which exerts force on the system even when it is at rest, as the load lifted by a crane or winch, a vehicle on a slope, or the like.

A further object is to provide an improved control arrangement for a hydraulic system wherein a hydraulic pump is connected to a hydraulic motor that displaces the load and .a brake is operatively connectable to the motor to lock the load in place.

SUMMARY OF THE INVENTION These objects are attained according to the present invention in a system wherein an auxiliary pump is connected to the high-pressure line of the hydraulic circuit between pump and motor and through a variable pressure-limiting valve to a reservoir. A sensor is provided to detect the back force exerted by the load, which is usually the vertical component of the mass of the load, and a control circuit is connected between this sensor and the pressure-limiting valve to pressurize the circuit to the extent necessary to balance the load hydrostatically before the control circuit releases the brake.

In accordance with the present invention another sensor is provided in the high-pressure line and the control circuit includes a comparator for releasing the brake when the two sensed forces are at levels indicating that the load will be hydrostatically balanced.

With this arrangement as the load is stabilized and when it is started from standstill the pressure limiter is activated with a voltage proportional to the resistance of the load and thereby reduces the portion of the output of the auxiliary pump that is sent to the reservoir. This causes the pressure in the high-pressure line to rise until it is at a level sufficient to hold the load at which point the brake is released automatically. Displacement of the load is effected by changing the pressure in the main hydraulic circuit so as to eliminate the hydrostatic equilibrium. This moves the load at a slow rate proportional to the extent of the imbalance. During this mode of operation any change of the mass of the load is automatically compensated by an adjustment of the voltage fed to the pressure limiter so that the slow displacement rate remains constant.

According to other features of this invention the hydrostatic transmission for carrying out the method according to the invention comprises a switch between the pressure limiter and the control circuit. This switch is normally open and is operated by the control element that the operator actuates to raise or lower the load. The control element is a lever displaceable in a T- shaped slot in two directions from the central T-leg,

each direction corresponding to a direction of displacement of the load and the extent of displacement of the lever from the central T-leg being directly proportional to the displacement rate. Displacement of the lever into the T-leg locks the load in place, whereas displacement out of this leg establishes the hydrostatic balancing before the brake is released and the load can be moved.

In accordance with another feature of the invention an extremely slow rate of displacement of the load is possible without the conventional irregular or jerky displacement speed by means of a transmission between the output of the motor and the load. The motor has an output shaft connected to one of the input shafts carrying the side gears of a differential whose cage carrying the differentials spider gears is connected directly to the load. The other input shaft of the differential is connected to an element which resists rotation of this shaft so that slow speeds of displacement of the load are achieved with a high rotation rate at the output shaft of the motor, with the difference being taken up by slippage in the differential. For higher speeds of displacement of the load, the other input shaft of the differential is locked so that the pinion-gear cage rotates with the input shaft connected to the motor.

DESCRIPTION OF THE DRAWING The above and other objects, features, and advantages will become more readily apparent from the following description, reference being made to the accompanying drawing in which:

FIG. 1 is a largely diagrammatic view of the system according to the present invention;

FIG. 1A is an alternative embodiment of a detail of FIG. 1;

FIG. 2 is a top I] of FIG. 1; and

FIGS. 3 7 are top views of alternative embodiments of the detail as shown in FIG. 2.

SPECIFIC DESCRIPTION The system according to the present invention comprises basically, as shown in FIG. 1, an axial-piston pump 2 of variable displacement connected through a high-pressure line 8 and a low-pressure line 7 to a constant displacement axial-piston motor 3. A continuously operating motor or engine 6 drives the pump 2 as well as its conventional boost pump 5. In addition, a pressure regulator 4 of conventional construction is provided between the lines 7 and 8.

The motor 6 also continuously operates an auxiliary pump 9 whose output line 12 is connected through a pressure limiting valve to a reservoir 10. In addition, this output line 12 is connected through a conduit 13 having a check valve 14 to the highpressure line 8 so that fluid can flow from line 12 to view taken in the direction of arrow line 8 when the pressure in the former is higher than h t in the tter. A

A control circuit 18 is connected via a line 26 to a pressure sensor 27 in the line 8 and via a wire 28 to an electric brake 29 carried on the pump 3, as also shown in FIG. 2. This pump 3 has a housing 68 carried on a plate 30 journaled on the output shaft 32 of the pump 3 on a transmission 35. This plate 30 is provided with a radially extending tab 69 which is connected via a short link 33 to a lug 34 also fixed to the transmission 35. The link 33 has a strain gauge that is connected via a wire 19 to the control circuit 18. In addition, the g 7 variable pressure limiter 15 is connected througha normally open switch 17 in a conductor 16 to the control s rqui 8.1

In this manner the control circuit 18 has two inputs 26 and 19 which respectively feed to it the pressure in line 8 and the amount of torque being exerted between the pump housing 68 and its shaft 32, since the brake 29 locks this shaft to the housing 68.

The output shaft 32 of pump 3 is connected through stepdown gearing 70 to the drive shaft 36 of a winch drum 25 around which is wound a cable 24 serving to lift a load 22 carried on a hook 23. The other end of the cable 24 may be connected via another link 48 provided with a strain gauge 46 to a fixed location 47. This latter strain gauge 46 can be used in place of the gauge 20 as it also detects the weight of the load 22.

The lifting and lowering of the load 22 is controlled by a single lever 37 displaceable in a T-shaped slot 71 between two terminal positions C and D corresponding to full-speed lift and full-speed drop, respectively, and a pair of central positions A and B corresponding to conditions of the load being stationary and either mechanically locked in place by brake 29 or hydrostatically balanced, respectively. In the position A the lever 37 opens a switch 39 which allows a relay 72 to release and open the conductor path 16 from the control circuit 18 to the pressure limiter 15. A pilot light 40 is provided between the switch 39 and relay 72 which indicates, when lighted, that the hydrostatic balancing system is engaged. In addition, the lever 37 acts effectively as the wiper along a resistive path 42 connected to a control circuit 43. A small tachometric sensor 45 is provided on the drum 25 and connected to the control circuit 43 which compares its two inputs and controls the swash plate of pump 2 via a control element 44 when the rotation speed sensed at 25 does not corre spond to the intended rotation speed set on lever 37. The resistor 42 is also connected to the control circuit 18.

This system functions as follows:

When the lever 37 is in position A the relay 72 is open and the pressure limiter 15 is disconnected from the control circuit 18 so that it opens up completely and drains all the output of pump 9 into the reservoir 10. In addition there is no fluid flow through the pump 2 as its switch plate is held by unit 44 in a position orthogonal to its axis of rotation. The automatic electronic control circuit 18 is indeed making readings at 20 and 27 which indicate that the load 22 is not hydrostatically balanced and is emitting at 16 a voltage to correct this, but the open circuit to the limiter 15 prevents this from being effective. The brake 29 is not energized in this position with voltage from the control circuit 18 so that it grips a rotor 73 on the pump shaft 32 to prevent this shaft 32 and the shaft 36 of the drum 25 from rotating. In this position the entire apparatus may indeed be shut down as the brake 29 when not energized prevents the load from dropping.

In order to move the load the operator first displaces the lever 37 into position B. This operation closes the switch 39 which in turn closes the relay 72 and allows the control circuit 18 to feed electricity to the limiter 15 to reduce flow through this element. Pressure therefore builds up in line 12 and passes through branch 13 and check valve 14 into the high-pressure conduit 8. The level of the voltage fed to the limiter 15 is proportional to the amount of force exerted by tab 69 on the strain gauge 20, thus it is proportional to the amount of torque that the load 22 is exerting on the shaft 32 which is locked .by brake 29 to housing 68. The greater the voltage fed to the limiter 15 the smaller the restriction formed by the limiter l5 and, hence, the greater the pressure in line 12. The pressure builds up in the line 8 and, when the sensor 27 gives to the control circuit 18 a reading indicating that this pressure is sufficient to overcome the level of torque read by strain gauge 20, the brake 29 is released. The load 22 istherefore balanced hydrostatically. Should it change weight, due for example to a wind blowing on a suspended load, the sensor 20 will detect the correspondingly changing torque and the control circuit will compensate for it through the limiter 15, so that even a changing mass is hydrostatically balanced. The output of the auxiliary pump therefore passes partially into the reservoir and partially into the line 8 where it compensates for leakage through the pump 2 whose output is still zero. The pressure in line 8 is constantly monitored by the I sensor 27 in order to keep it at the level necessary for the motor 3 to overcome the torque sensed at 20.

Displacement of the lever 37 slightly toward the position C and to a location C only serves to close the limiter further so that the pump 9 alone functions to raise the load. Since this pump 9 is of relatively uniform and small output it therefore serves to raise the load 22 at a very slow and uniform rate, a speed extremely useful in the careful positioning of delicate loads and hitherto unobtainable since the large pump 2 when operated with a very flat angle of its swash plate gives a nonuniform output.

Similar short displacement of the lever 37 toward the position D to a region D opens the valve 15 slightly wider than necessary to balance the load 22 statically so that this load 22 will drop at a very slow and steady rate.

A more rapid raising or lowering of the mass 22 is effected by displacing the lever 37 beyond positions C and D" respectively. Such positioning of the lever furnishes a voltage to the control circuit 43 which is proportional to the speed desired. The control circuit 43 in turn compares this set point from the resistor 42 with the controlled variable from tachometric dynamo 45 and generates a corrective signal which operates the servomotor 44 while closing the limiter l5 altogether so as to maintain at 45 a rotation speed which gives the load 22 the desired lifting or lowering speed.

FIG. 1A shows an alternative arrangement wherein instead of the tie-rod 33 the tab 69 is connected to the lug 34 through a short hydraulic cylinder 33 equipped with a pressure gauge that is in turn connected to the control circuit 18.

Most hydraulic motors have a particular rotation speed of highest efficiency, and in the axial-piston type particularly operation below this speed causes the output shaft of the motor to turn with a nonuniform angular velocity. Clearly this is undesirable in a crane or the like where it is often necessary to move loads slowly and at a uniform speed. Thus as shown in FIG. 3 means is provided which allows the motor 3 to operate at a higher speed while still driving the winch drum at a very slow rate.

.To this end the output shaft 32 of the motor 3 serves as one of the input shafts carrying a side gear of a differential 50 whose pinion gears are carried on a cage 52 carrying the ring gear 74 meshing with a gear train driving the drum 25. The stepdown transmission here has a central shaft 53 on which the brake 29 is mounted. The other input shaft 54 of the differential carries a gear 76 meshing with a pair of gears 77 and 78. A shaft 75 carries the gear 78'and is provided with an electric brake 55 operated by the control circuit 18. The other gear 59 is connected to a conventional hydraulic torque converter 56 whose housing 57 is secured to the transmission 35.

This arrangement operates as follows: As long as the lever 37 is between positions C' and D the brake 55 is released. Thus the differential 50 slips with the slippage being taken up by the torque converter 56. Since the turbine of this converter 56 is fixed the shaft 54 of the differential can only rotate with a certain maximum speed. In this manner the motor 3 will operate in a range wherein its output is uniform, with the difference between this output speed and the desired speed being taken up by slippage in the differential 50. Once, however, a speed of displacement of the drum 25 is desired at which the motor 3 can operate smoothly the brake 55 is closed by the control circuit 18 and the shaft 54 of the differential 50 is arrested. This locks the shaft 32 and the cage 52 together for direct driving of the drum 25. In addition it is possible with this system to warm up the motor 3 by operating it with the brake 29 locked, in which case the shaft 54 will rotate in a direction opposite to that of the shaft 32 and at the same angular rate. During such warmup care is taken not to exceed the speed at which the converter 56 can no longer compensate by slipping. In this arrangement also the sensor 46 is used instead of the sensor 20 as the back force appearing as torque on the motor 3 is not indicative of the weight of the load 22. The brakes 29 and 55 are generally connected to front and back poles of a single relay so that only one of them can be operated at a time to prevent damage to the transmission 35, although at times' both brakesmay be released.

The arrangement of FIG. 4 is identical to that of-FIG.

' 3 except that the turbine 57 of theconverter 56 is rotatable and carried on a motor 58 which is driven at a speed to regulate the displacement rate of the drum 25 as exactly as possible.

In FIG. 5 the converter 56 is replaced by an electric clutch 60 having a central winding 62 and a surrounding rotor 79. Energization of the winding 62 impedes rotation of the element 79 to an extent proportional to the strength of the magnetic field created by this winding 62.

FIG. 6 shows an arrangement similar to that of FIG. 5 except that the clutch 60 is carried on a motor 63 such that a very sensitive control of the rotation speed is possible, much as in FIG. 4.

In FIG. 7 the shaft 59 drives a positive displacement hydraulic pump 64 which is connected via a line 67 to a reservoir 65. The high-pressure side of the line 67 is provided with a solenoid-type pressure limiting valve 66 much the same as valve 15. The back pressure against motor 64 therefore determines how fast it may rotate, which in turn determines the slippage in the differential ,50. Very exact control of the relative speeds of shafts 32 and 54 is achieved to allow the drum 25 to be turned very slowly with a very uniform speed.

Any of the alternatives of FIGS. 3 7 can be substituted into the arrangement of FIG. 1. The hoist according to this invention allows a load regardless of weight to be hydrostatically balanced and moved at a very slow rate. The balancing is accomplished automatically, and even functions with, for instance, a bucket which is being emptied to hold it at a constant level hydrostatically.

I claim:

1. A hydraulic system for displacing a load, comprisa lifting element operatively connected to said load;

a hydraulic motor operatively connected to said lifting element;

a variable-displacement hydraulic pump;

high-pressure and low-pressure hydraulic lines interconnecting said pump and said motor;

means for varying the displacement of said pump and thereby displacing said load at a rate proportional to the displacement of said pump;

brake means operatively connected to said lifting element for arresting said load;

an auxiliary hydraulic pump connectable to said high-pressure line;

pressure sensing means for detecting the hydraulic pressure in said high-pressure line;

force sensing means operatively connected to said lifting element for detecting the force said load exerts thereon; and

electronic control means connected between said sensing means and said brake means for releasing said brake means when said pressure in said highpressure line is sufficient to overcome said force.

2. The system defined in claim 1, further comprising a reservoir and valve means connected to said control means and connected between said auxiliary pump and said reservoir for diverting a portion of the output of said auxiliary pump into said reservoir.

3. The system defined in claim 2 wherein said valve means includes a first hydraulic line between said auxiliary pump and said reservoir, a pressure-limiting valve in said first line, a second line between said first line and said high-pressure line and connected to said first line between said valve and said auxiliary pump, and a check valve in said second line permitting fluid flow therethrough toward said high-pressure line.

4. The system defined in claim 3, further comprising switch means connected to said pressure-limiting valve for disconnecting same from said control means and maintaining said valve in a fully open condition.

5. The system defined in claim 4, further comprising a control element connected to said control means and to said means for varying the pump displacement and displaceable between a first position wherein said switch is open and said brake means is applied, a second position wherein said switch is closed, and third and fourth positions corresponding to opposite directions of displacement of said element.

6. The system defined in claim 1 wherein said motor has a rotor operatively connected to said element and a stator, said system further comprising a fixed support pivotally carrying said stator and a link between said support and said stator for preventing pivoting thereof relative to said support, said force sensing means comprising a sensor at said link for detecting the torque said stator exerts on said support through said link.

7. The system defined in claim 1, further comprising a differential having a pair of input shafts and an output, and means for resisting rotation of and arresting one of said input shafts, the other input shaft being operatively connected to said motor and the output being operatively connected to said element.

8. The system defined in claim 7 wherein said means for resisting is a torque converter.

9. The system defined in claim 7 wherein said means for resisting is an electrical clutch.

10. The system defined in claim 7 wherein said means for resisting includes a hydraulic motor and means for limiting fluid flow therethrough.

l l. The system defined in claim 7 wherein said means for resisting includes a motor connected to said control means and operable to rotate said one input shaft at a predetermined speed.

12. A method of operating a hydraulic system for displacing a load comprising a variable-displacement pump connected through highand low-pressure hydraulic lines to a hydraulic motor operatively connected to said load, and a brake for preventing displacement of a lifting element displaced by said motor to move said load, said method comprising:

feeding fluid into said high-pressure line from an auxiliary pump;

detecting the amount of force exerted by said load on said lifting element;

detecting the hydraulic pressure in said high-pressure line; and

releasing said brake when the pressure detected in said high-pressure line is high enough to overcome the force detected on said lifting element.

13. The method defined in claim 12 wherein said fluid is fed into said line by diverting same from a conduit extending between said auxiliary pump and a reservoir.

14. The method defined in claim 13 wherein said fluid is diverted by connecting said conduit to said high-pressure line and placing a restriction in said conduit.

15. The method defined in claim 1, further comprising the steps of operating said motor at a speed faster than a minimum speed at which it displaces said lifting element at a nonuniform rate and allowing slippage between the output of said motor and said lifting element. =i 

1. A hydraulic system for displacing a load, comprising: a lifting element operatively connected to said load; a hydraulic motor operatively connected to said lifting element; a variable-displacement hydraulic pump; high-pressure and low-pressure hydraulic lines interconnecting said pump and said motor; means for varying the displacement of said pump and thereby displacing said load at a rate proportional to the displacement of said pump; brake means operatively connected to said lifting element for arresting said load; an auxiliary hydraulic pump connectable to said high-pressure line; pressure sensing means for detecting the hydraulic pressure in said high-pressure line; force sensing means operatively connected to said lifting element for detecting the force said load exerts thereon; and electronic control means connected between said sensing means and said brake means for releasing said brake means when said pressure in said high-pressure line is sufficient to overcome said force.
 2. The system defined in claim 1, further comprising a reservoir and valve means connected to said control means and connected between said auxiliary pump and said reservoir for diverting a portion of the output of said auxiliary pump into said reservoir.
 3. The system defined in claim 2 wherein said valve means includes a first hydraulic line between said auxiliary pump and said reservoir, a pressure-limiting valve in said first line, a second line between said first line and said high-pressure line and connected to said first line between said valve and said auxiliary pump, and a check valve in said second line permitting fluid flow therethrough toward said high-pressure line.
 4. The system defined in claim 3, further comprising switch means connected to said pressure-limiting valve for disconnecting same from said control means and maintaining said valve in a fully open condition.
 5. The sYstem defined in claim 4, further comprising a control element connected to said control means and to said means for varying the pump displacement and displaceable between a first position wherein said switch is open and said brake means is applied, a second position wherein said switch is closed, and third and fourth positions corresponding to opposite directions of displacement of said element.
 6. The system defined in claim 1 wherein said motor has a rotor operatively connected to said element and a stator, said system further comprising a fixed support pivotally carrying said stator and a link between said support and said stator for preventing pivoting thereof relative to said support, said force sensing means comprising a sensor at said link for detecting the torque said stator exerts on said support through said link.
 7. The system defined in claim 1, further comprising a differential having a pair of input shafts and an output, and means for resisting rotation of and arresting one of said input shafts, the other input shaft being operatively connected to said motor and the output being operatively connected to said element.
 8. The system defined in claim 7 wherein said means for resisting is a torque converter.
 9. The system defined in claim 7 wherein said means for resisting is an electrical clutch.
 10. The system defined in claim 7 wherein said means for resisting includes a hydraulic motor and means for limiting fluid flow therethrough.
 11. The system defined in claim 7 wherein said means for resisting includes a motor connected to said control means and operable to rotate said one input shaft at a predetermined speed.
 12. A method of operating a hydraulic system for displacing a load comprising a variable-displacement pump connected through high- and low-pressure hydraulic lines to a hydraulic motor operatively connected to said load, and a brake for preventing displacement of a lifting element displaced by said motor to move said load, said method comprising: feeding fluid into said high-pressure line from an auxiliary pump; detecting the amount of force exerted by said load on said lifting element; detecting the hydraulic pressure in said high-pressure line; and releasing said brake when the pressure detected in said high-pressure line is high enough to overcome the force detected on said lifting element.
 13. The method defined in claim 12 wherein said fluid is fed into said line by diverting same from a conduit extending between said auxiliary pump and a reservoir.
 14. The method defined in claim 13 wherein said fluid is diverted by connecting said conduit to said high-pressure line and placing a restriction in said conduit.
 15. The method defined in claim 1, further comprising the steps of operating said motor at a speed faster than a minimum speed at which it displaces said lifting element at a nonuniform rate and allowing slippage between the output of said motor and said lifting element. 